The present invention relates to an accelerometer and, more particularly, to an improved force balance accelerometer of simple, reliable construction.
Devices for measuring acceleration are used in various types of test equipment and, also, as components in numerous systems. In the simplest type of accelerometer, a mass is spring mounted on a reference frame. When the mass experiences acceleration, a force resulting from the acceleration moves the mass with respect to the frame by a distance related to the magnitude of the force. Measurement of the displacement of the mass from a reference position thus provides an indication of the magnitude of the acceleration in the direction of displacement. One such accelerometer, disclosed in U.S. Pat. No. 4,353,259, issued Oct. 12, 1982, utilizes a fiberoptic fiber as the mass, with the fiber being held at only one end. The opposite end of the fiber is free to deflect, and the amount of deflection, and therefore the magnitude of the deflecting acceleration, is determined by a plurality of appropriately positioned light detectors. Another such accelerometer is shown in U.S. Pat. No. 4,315,433, issued Feb. 16, 1982, to Edelman et al. The Edelman et al accelerometer utilizes a mass which is held between two sheets of piezoelectric polymer film. Deflection of the mass from a reference position is measured by changes in the piezoelectric properties of the film as the film is distorted in shape.
Such relatively simple accelerometers, open loop in operation, are subject to errors from a number of sources, including manufacturing errors, and errors resulting from thermal expansion of components. A more accurate type of prior art accelerometer has been developed which compensates for a number of these errors. Known as a force balance accelerometer, this type of device includes a proof mass which is mounted for free movement, usually pivotal movement, in response to experienced acceleration. An electromagnetic arrangement applies a restoring force to the proof mass which is sufficient to return it to a reference position. The level of current required to produce the restoring force provides a measure of the acceleration experienced by the proof mass.
Several types of mounting arrangements have been utilized with force balance accelerometers. U.S. Pat. No. 3,323,372, issued June 6, 1967, to Kistler et al discloses a device having a plurality of support wires which extend from the proof mass to a mounting frame. U.S. Pat. No. 2,947,176, issued Aug. 2, 1960, to Perry discloses an accelerometer having the proof mass mounted on a pair of leaf springs. U.S. Pat. No. 3,339,419, issued Sept. 5, 1967, to Wilcox, relates to an accelerometer having a proof mass supported by a cantilever groove arrangement which permits the somewhat thinner material in the region of the groove to flex.
It will be appreciated, however, that the above described mounting arrangements are all somewhat undesirable in that the restoring force applied to a deflected proof mass is a function both of the electromagnetically generated force and the spring constant of the mounting arrangement for the proof mass. Other devices, such as shown in U.S. Pat. No. 4,495,815, issued Jan. 29, 1985, to Stratton et al, utilize pivot bearing structures to mount the proof mass, which bearings eliminate the application of force to the proof mass by the spring mounting. Bearing arrangements, such as ball bearings and jeweled bearing mounts which have been utilized with prior art systems, however, are subject to wear and friction, and additionally are relatively expensive.
Accordingly, it is seen that there is a need for an improved accelerometer of the force balance variety which is simple and economical in construction, and reliable in operation.